You`re sitting in a web-backed plastic chair on a blue metal deck, gliding on still waters past a house Frank Sinatra once considered buying. Or so says Bob, your guide aboard the Jungle Queen.

``There`s the telephone number on the for sale sign,`` Bob points out as you pass. ``It`s got 19,000 square feet of living space. Seven bedrooms and seven baths.``

You`ll see a lot more as you head down the Intracoastal Waterway and along the New River in Fort Lauderdale. To your left, the home of Truly Nolen, exterminator extraordinaire. Note the blue tile roof and the yellow kiddie cars with mouse ears and tails on the patio. To your right, a sailboat once owned by the son-in-law of Benito Mussolini. Straight ahead, a five-acre island where an Indian named Ron will wrestle a alligator without breaking a sweat.

And all around you -- and the rest of South Florida -- behold one of the area`s biggest small businesses: sightseeing and dinner cruises that travel the area`s inland waters.

The Jungle Queen, based in Fort Lauderdale for 45 years, is one of the largest and best-known local cruise lines. But about a dozen others, most of them privately held local companies, are competing for a lucrative market that has grown from typical tourist fare to a year-round circuit of weddings, bar mitzvahs and business meetings.

``Celebrating on the water is festive,`` said Raleigh Perry, sales director at Fandango Cruises Inc., Fort Lauderdale. ``You have a good time, you pay your money and leave us with the headaches.``

These days, more locals have learned what tourists could have told them years ago: It`s fun riding someone else`s boat. Where else can you cruise past the current or past homes of Julio Iglesias and Al Capone, drink in hand, cool breeze on your face?

Although many local cruise lines, such as the Jungle Queen and the Paddlewheel Queen, have made a living from tourism, a lot of companies now are changing their focus to include more hometown trade. Chartering a boat for special occasions can cost about the same as renting a reception hall and hiring a caterer, a bartender and a band.

``We position ourselves as a sophisticated Florida nightclub,`` said Robert F. Lambert, president of Florida Princess Cruise Lines, Fort Lauderdale. ``This is not just a sightseeing boat.``

The Florida Princess is more like a fancy floating restaurant: It has carpeting, three bars -- one paneled in mahogany -- a bandstand and a hot tub. It`s worth more than $1 million.

Of course, such an atmosphere carries a price: A wedding aboard Fandango, for instance, can cost $3,000 to $20,000, if someone wants to go overboard. And people are willing to pay.

``Over the summer months, we had at least one wedding a week,`` Perry said. ``Some weekends we have several.``

Cruise companies also offer lunch jaunts -- often to Miami`s Bayside Marketplace -- moonlight dinner cruises and Sunday brunches that run from $9.95 to $25.95. Food is one of the biggest sources of revenue.

``Souvenirs are a very small part,`` said Jerome Faber, owner of the Jungle Queen. ``We do millions in terms of our night business with the dinner cruises and the shows.``

The fact that Florida`s weather is kind to cruises year-round makes the market one of the nation`s best.

Still, cruise directors say, it`s often rough sailing.

Upkeep on boats that size -- usually 70 to 130 feet long -- is expensive. Keeping the Florida Princess ship-shape, Lambert said, costs $150,000 a year.

Meeting Coast Guard regulations and buying insurance, training a crew, paying cooks and finding waiters who also can sing and dance in on-deck shows also drain profits.

Although the year-old Fandango will attract an average of 300 customers a day and gross $2 million in 1987, the business will just break even, says company President Robert Buckley.

``It takes two to three years to build up a business of this sort,`` Buckley said. ``If you do it slowly, it could be OK.``

Even the world-famous Jungle Queen still races to compete. The company recently bought Nikko Gold Coast Cruises in Miami to expand its South Florida market and boost its number of boats to five, more than many local lines.

``Anyone who operates a ship down here is our competition,`` Faber said.

Increasingly, out-of-town cruise lines are part of that competition. The start of the tourist season next month means ships that cannot cruise in the cold Northeast will spend their winters in South Florida -- and take business away from the locals.

``It`s a problem for our industry,`` said Capt. Jim Cross, owner of Island Queen Excursions Inc. in Riviera Beach. ``People in business on the East Coast are looking for money in the winter, and they usually come down and take some of the money from us.``

``What it does is muddy up the water,`` said Fandango`s Buckley. ``Just to pay wages for the winter, they`ll cut prices and cut quality and just mess up the order of things for three or four months.``